With the increasing demand for high-rate, high bandwidth efficiency data communication systems, high order modulation techniques such as high order pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) have gained attention in recent years. The advantage of higher order modulation is the possibility to transmit more bits per symbol. The data modulated by such higher order modulation techniques, however, becomes more susceptible to inter-symbol interference.
The inter-symbol interference is caused by nonlinear effects in both electrical and optical domains, such as drivers, amplifiers, modulators, and fibers. To avoid system degradation caused by the inter-symbol interference, equipment manufacturers and network operators are continually looking for ways of compensating for the nonlinear effects.